The present invention relates to a malfunction detector for an oil-fuel heat exchanger used in the lubrication circuit of an aircraft gas turbine engine.
Fire on board an aircraft represents a most critical occurrence, one which may easily result in catastrophe. A modem gas turbine aircraft engine requires the use of flammable materials, such as fuel and lubricating oil. It is imperative, therefore, to preclude leaks of these flammable materials, which may be occasioned by poor seals and degradation of mechanical parts in the fuel and lubricating systems in order to prevent such fluids from contacting high temperature parts of the gas turbine engine.
Most turbojet engines are equipped with an oil-fuel heat exchanger mounted in the lubrication circuit. When the heat exchanger fails, fuel and oil from the two fluid circuits may intermix which, in most cases, will involve the flow of fuel from the fuel circuit into the lubricating oil circuit due to the higher fuel pressure within the heat exchanger.
The intermixing of the fuel and oil circuits are manifestly undesirable. Dilution of the oil with the fuel substantially increases the danger of an internal engine fire, especially when the aircraft is on the ground after operation, since the labrynth seals of the aircraft engine may be diminished and the engine temperature is great enough to cause ignition of the fuel and the oil. Moreover, the fuel degrades the lubricating properties of the lubricating oil, which may further raise the temperature of the moving parts of the engine. The addition of the fuel to the lubricating circuit will cause the lubricating oil reservoir to overflow when the reservoir is at atmospheric pressure. If the reservoir is located within an enclosure, the pressure within the enclosure increases and the enclosure may be flooded.
U.S. Pat. No. 4,163,391 describes a device to monitor the level of the liquid in a reservoir using a high-temperature coefficient resistor whose resistance changes as a function of its temperature. This device compares several voltages and requires the use of a calibration and computation chart or graph, in particular to calculate the operating time of a delay system.